Process for heat treating metals



United States Patent 3,526,551 PROCESS FOR HEAT TREATING METALS HerbertSargent, 3416 Via Palomino, Palos Verde Estates, Calif. 90274 NoDrawing. Filed Jan. 17, 1968, Ser. No. 698,395 Int. Cl. C21d 1/68; C22f1/04, 1/08 U.S. Cl. 148-131 Claims ABSTRACT OF THE DISCLOSURE BACKGROUNDOF INVENTION In the heat treatment of sheets or bars of some metals andalloys, it has long been noted that there is a bonding or sticking ofthe surfaces of the metals to each other after cooling. Normally thisdoes not create a problem with sheets or bars since the surfacecharacteristics of these articles are not usually critical. When thearticle being heat treated, however, must have a highly polished surfaceor surfaces or when the article is a thin section which is easilydeformed, this bonding of the surfaces creates a serious problem.

One area in which this problem is particularly acute is in the heattreatment of thin aluminum alloy memory discs for use in commuters. Thesensitivities of the computer operation require that the disc surfaceshave an extremely high polish. Any irregularity created on the discsurfaces due to sticking during heat treatment renders the discsunusable so that they must be discarded as scrap. These discs arenormally thermally stress relieved between the initial machining wherethe disces are machined to about 14 to 36 inches in diameter and to 4inches in thickness and the finishing or polishing. With theconventional heat treating processes, these discs cannot be stackedwithout a high percentage of scrap being formed. Often if the discs arestacked, all of them must be discarded because the discs stick to eachother and become deformed during separation.

Thus, a process is needed for heat treating metal articles, such as thecomputer memory discs, at elevated temperatures so that they can bestacked or placed contiguously to other articles in an oven withoutdanger of sticking or bonding to other articles in the stack.

SUMMARY OF INVENTION This invention is directed to a process for heattreating metallic articles such as metallic sheets, bars and memorydiscs for use in computers. The process basically oomprises precoatingeach article with a layer of an organic compound having a hydroxylfunctional group. After the articles are coated, they are stacked incontiguous relation with each other and heat treated at temperaturesabove 800 F. to relive machining stresses set-up in the articles duringtheir formation. After the heat treating of the articles with thecoating of organic compound on the surfaces thereof, they can easily beseparated by hand without the need for prying the articles apart orscraping or gouging their surfaces.

The coating material which is applied to the surfaces of the articlesmay be any organic compound having a ice hydroxyl functional group suchas the alcohols including the monohydric alcohols; the dihydric alcoholsor the diols or glycols; and the polyhydric alcohols or polyols orpolyglycols. The glycols and polyglycols are preferred because they areless volatile than the monohydric alcohols and they have a greaternumber of hydroxyl functional groups. The compound which has been foundto give the most staisfactory results is propylene glycol(1,2-propanediol).

The metals to which this process is applicable are aluminum and aluminumalloys, magnesium and magnesium alloys, carbon steels and any othermetals which are bonded or stick to the surfaces of similar metals uponstress relieving or other heat treatment at elevated temperatures. Thetemperature employed for such heat treatment is usually as high aspracticable, but not high enough to impair the structure produced by anyprevious heat treatment. For example for steel the thermal stressrelieving is normally performed at temperatures just below thetransformation range. For aluminum alloys thermal stress relieving isgenerally performed at from about 400 F. to about 850 F.

A feature of the heat treating process of this invention is that metalarticles can be stacked one on top of the other or moved into contiguousrelationship with the surfaces of similar metal articles in the heattreating oven when heated to elevated temperatures such as stressrelieving temperatures without bonding or sticking of the articles toeach other so that force must be applied for their separation. Forexample, when aluminum memory discs are heat treated according to thisprocess in stacks in the oven, they can be easily hand separated fromeach other with no marks, scratches or other blemishes being formed onthe surfaces of the discs. Thus the percentage of scrapped or discardeddiscs after the heat treating process is dramatically reduced.

DESCRIPTION OF PREFERRED EMBODIMENTS AND DISCUSSION The preferred heattreating process of this invention is to brush coat aluminum alloy discswith propylene glycol to form a thin layer of the propylene glycol onall surfaces of the discs. The discs are then placed in stacks of atleast twenty-five at a time in a heat treating oven and heated to atemperature of above 400 F. The discs are maintained within the heattreating oven for a sufficient time to stress relieve the aluminumalloy. This treatment should be at least 15 minutes and preferably 30minutes. The discs are then slowly cooled by turning the heat treatingoven off and allowing them to remain in the oven until it has cooled.This may require leaving the discs in the oven 24 hours. It has beenfound that when the discs are so precoated there is no sticking orbonding between the discs and they can be easily separated after theheat treatment.

It has been determined that any organic compound having a hydroxylfunctional group will produce a reduction in the sticking or bonding ofthe adjacent metal surfaces during the thermal stress relievingtreatment. The preferred compounds, however, because they are readilyavailable in liquid form and easy to work with are alcohols includingthe monohydric alcohols, the glycols and the polyglycols such as methylalcohol, ethyl alcohol, butyl alcohol, isobutyl alcohol, secondary butylalcohol, t-butyl alcohol, lauryl alcohol, vinyl alcohol, allyl alcohol,ethylene glycol, diethylene glycol, triethylene glycol, propyleneglycol, polyethylene glycol, glycerin and mixtures thereof. The bestresults have been obtained with propylene glycol as the protective layeralthough it has been found that all of the monohydric and polyhydricalcohols tested have produced the desired effect.

Some of these compounds such as propylene and dipropylene glycol, havethe added advantage that they can be used as a cutting oil during theinitial machining of the discs or other articles so that it is notnecessary to thoroughly wash the articles prior to heat treatmentthereof. Some of the more highly volatile alcohols such as methyl andethyl alcohol are not as satisfactory since they evaporate too quicklyand thus must be applied immediately prior to heat treating in order toproduce the beneficial results.

The monohydric alcohols, in general, have been found to functionsatisfactorily in preventing sticking of the adjacent contiguousarticles in a stack during the heat treating thereof but the polyhydricalcohols and particularly the glycols due to their higher viscosity andlower volatility have been found to be even more satisfactory. Thehydroxyl functional group in some manner acts to prevent bonding of thecontiguous metal surfaces during the thermal treatment.

The mechanism by which organic compounds with hydroxyl functional groupsprevent sticking of contiguous metal surfaces during the heat treatingprocess has not yet been determined. It has been conjectured howeverthat the hydroxyl radical may combine with the metal to form a compoundsuch as a metal-organic salt which prevents migration of metallic atomsfrom one surface to another and therefore prevents bonding of themetallic surfaces to each other.

Most of the development work for this process has been done on thecopper-aluminum, magnesium-aluminum, and zinc-aluminum precipitationhardenable alloys designated 7075, 6061, 5083, 5086 and 2024 since thesealloys have applicability for use as computer memory discs. The processin its broader aspect is applicable to any metals which are bonded orstuck together during heat treatment at stress relieving temperaturessuch as magnesium alloys, zinc alloys, carbon steels etc.

This invention will be more clearly understood from a consideration ofthe foregoing discussion when taken in conjunction with the followingspecific examples.

EXAMPLE 1 A solution of 80% by volume lauryl alcohol and by volumemyristyl alcohol was prepared. This solution was added to diisobutylalcohol to form a treating solution of by volume diisobutyl alcohol and75% by volume of the lauryl and myristyl alcohol mixture. A set oftwenty-five 7075 aluminum alloy discs having a nominal composition of.5% Si, .7% Fe, 1.22.0% Cu, .3% Mn, 2.1-2.9% Mg, .18.40% Cr, 5.16.1% Zn,.2% Ti and less than .15 others were machined to a thickness of inch anda 14 inch diameter and then brush coated with the alcohol solution andstacked one on another in an annealing oven. The temperature of theannealing oven was brought to 500 C. and the discs were held at thattemperature in the oven for minutes in an air atmosphere. These discswere then slowly cooled to room temperature, removed from the oven, andhand separated. No appreciable sticking or bonding of the discs wasnoted.

EXAMPLE 2 A set of twenty-five 14 inch diameter and inch thick 7075aluminum alloy discs which had been machined to a fairly high surfacepolish were brush coated with a solution of butyl Cellosolve (ethyleneglycol monobutyl ether). These discs were then stacked in a single stackin an annealing oven and annealed at a temperature of 700 F. The discswere then slowly cooled to room temperature, removed from the oven, andhand separated. No appreciable sticking or bonding of the discs to oneanother was noted.

EXAMPLE 3 A set of twenty-five 2024 aluminum alloy discs having anominal composition of 5% S .5% Fe, 3.84.9% Cu,

.30.90% Mn, 1.21.8% Mg, .1% Cr, 15% Zn and a total of less than .15%others were machined to a thickness of about inch, a 14 inch diameterand a fairly high surface polish. The discs were brush coated with alayer of triethylene glycol. The discs were then stacked in a singlestack in an annealing oven and heat treated at a temperature of 850 C.for 30 minutes. The discs were then slowly cooled by allowing them toremain in the oven for 24 hours after the heat had been turned off,removed from the oven, and hand separated. No appreciable sticking orbonding of the discs was noted.

EXAMPLE 4 A set of twenty-five 2024 aluminum alloy discs were machinedto a thickness of about inch and a 14 inch diameter with a fairly highsurface finish. These discs were brush painted by hand with a layer ofpropylene EXAMPLE 5 A set of twenty-five 2024 aluminum alloy discs weremachined to a thickness of about A inch and a 14 inch diameter with afairly high surface polish. These discs were then painted with a brushwith a polyethylene glycol 200 solution to form a thin coating on thesurfaces of the discs. The discs were then inserted into a heat treatingfurnace in a single stack and heated to a temperature of 800 F. for atleast 30 minutes. The discs were then cooled in the oven, removed fromthe oven and hand separated. No appreciable sticking or bonding of thediscs was noted.

EXAMPLE 6 A set of twenty-five 5086 aluminum alloy discs having anominal composition of .4% si, .5 Fe, .1% Cu, .2-.7% Mn, 35-45% Mg,.05-.25% Cr, 25% Zn, .15% Ti and less than .15 total others weremachined to a 14 inch diameter and a inch thickness with a fairly highsurface polish. A thin coating of glycerin was applied by means of abrush to all surfaces of the discs and they were placed in a heattreating oven in a single stack and heated to a temperature of 750 F.for 30 minutes to stress relieve the discs. After this stress relieftreatment, the discs were then cooled in the oven for about 24 hours,removed from the oven and hand separated. No appreciable sticking orbonding of the discs was noted.

The discs coated with propylene glycol exhibited a much cleaner surfaceafter the heat treatment than the discs coated with other treatingmaterials. All of the discs, however, exhibited much cleaner surfacesafter the treatment with an organic compound having the hydroxylfunctional group than discs which were heat treated without a precoatingwith a compound having hydroxyl functional groups. For example, whenkerosene was used to precoat the discs, all of the discs had to bediscarded as a result of sticking during the heat treating process. Whenno coating at all was used on the discs, it was customary to scrap ashigh as of those discs heat treated if the surfaces of the discs were incontact with each other.

While the invention has been described with partiular reference to thethermal stress relief of aluminum alloys it is more broadly applicableto the treatment of any metals which become bonded to similar metalsurfaces during the heat treatment thereof. The present embodiments aretherefore to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than by the foregoing description, and all changes whichcome within the meaning and range of equivalency of the claims aretherefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:

1. A process for heat treating metal articles such as sheets, rods anddiscs comprising the steps of:

coating said articles with an organic compound having a hydroxylfunctional group;

placing said articles in a heat treating oven so that surfaces of saidarticles are in contact with each other;

heating said articles to a temperature sufiiciently high to stressrelieve said articles;

cooling said articles to room temperature in a stress relievedcondition; and

separating said articles from each other.

2. A process as defined in claim 1 wherein said compound having ahydroxyl functional group is an alcohol.

3. A process as defined in claim 1 wherein said compound having ahydroxyl functional group is a polyhydric alcohol.

4.. A process as defined in claim 1 wherein said compound having ahydroxyl functional group is a glycol.

5. A process as defined in claim 1 wherein said compound having ahydroxyl functional group is propylene glycol.

6. A process as defined in claim 1 wherein said coating step comprisesapplying said compound to the surfaces of said articles with a brush.

7. A process as defined in claim 1 wherein said articles are heated to atemperature of from 400 F. to just below the melting point of the metalbeing treated and maintained at said temperature for at least 15 minutesto relieve internal stresses developed during forming of said articles.

8. A process as defined in claim 1 wherein said articles comprisealuminum alloy memory discs for use in computers.

9. A process as defined in claim 1 wherein said compound is an alcoholhaving a sufficiently high viscosity to be brush coated on the surfacesof said articles and a sufficiently low volatility so that said alcoholremains on the surfaces of said articles while said articles are beingstacked in said heat treating furnace.

10. A process as defined in claim 1 wherein said articles comprisealuminum alloy discs; said discs are heated to a temperature of from 400F. to 850 F. and said discs are slowly cooled to room temperature byturning off said oven and allowing said discs to remain in said oven forabout 24 hours while said oven cools.

References Cited UNITED STATES PATENTS RICHARD O. DEAN, Primary ExaminerU.S. Cl. X.R.

